Questions: Thermobarometry and Pressure-Temperature Paths

Question 1

Why is the Fe/Mg ratio in coexisting garnet and biotite useful as a geothermometer?

• A. Iron and magnesium have very different atomic masses, so their ratio changes predictably as rock density increases with depth
• B. The partitioning of Fe and Mg between the two minerals is temperature-dependent: the equilibrium Fe/Mg ratio in each mineral shifts in a known way with temperature and has been experimentally calibrated
• C. Garnet always contains more iron and biotite always contains more magnesium regardless of temperature, so their ratio reflects pressure rather than temperature
• D. Iron and magnesium undergo radioactive decay at different rates, so their ratio acts as a radiometric clock recording crystallization age

Answer: (B)

Geothermometers rely on element partitioning: how an element distributes itself between two coexisting phases depends on temperature. For garnet-biotite, more Mg enters garnet (and more Fe enters biotite) at higher temperatures, shifting the KD = (Fe/Mg)garnet / (Fe/Mg)biotite value in a calibrated, reproducible way. By measuring the ratio in both minerals from the same rock and applying the experimental calibration, you calculate the temperature at which they last equilibrated. This is exchange thermometry — no radioactive decay involved (option D is a different technique: geochronology).

Question 2

A metamorphic rock preserves a garnet crystal whose core records high pressure and moderate temperature, while the rim records lower pressure and higher temperature. What tectonic setting and history does this P-T path suggest?

• A. A counterclockwise path, indicating the rock was heated by a nearby igneous intrusion before being buried
• B. A clockwise P-T path consistent with deep burial during continental collision (high P), followed by heating during exhumation and then cooling at the surface
• C. A simple burial path with no uplift — the core formed deeper and the rim formed shallower as the crust compressed
• D. An isothermal path indicating the rock was transported horizontally through the crust without significant heating or cooling

Answer: (B)

A sequence of high pressure → lower pressure + higher temperature describes a clockwise P-T path: rapid burial to depth (increasing pressure), then heating at depth (temperature rises as the rock equilibrates with the geotherm), then exhumation (pressure decreases while temperature may briefly continue rising due to slow thermal equilibration). This is the hallmark of continent-continent collision zones like the Himalaya. A counterclockwise path (option A) would show heating before pressure increase, characteristic of contact metamorphism near intrusions. The core-to-rim chemical evolution records this path over millions of years.

Question 3

A P-T-t path records not just the pressures and temperatures a rock experienced during its history, but also the rate at which it moved through those conditions.

• T. True
• F. False

Answer: True

P-T paths from thermobarometry are combined with 't' (time) from thermochronology — radiometric systems that close at specific temperatures, giving cooling ages. Rapid cooling (many °C per million years, constrained by two or more thermochronometers at different closure temperatures) implies fast exhumation by faulting or erosion. Slow cooling implies gradual uplift or thermal relaxation. The rate dimension is what converts a geometric path in P-T space into an actual tectonic history with rates of burial, heating, and exhumation.

Question 4

Thermobarometry determines the closure temperature of a rock's radiometric system — the temperature at which isotopes stopped diffusing — making it equivalent to thermochronology.

• T. True
• F. False

Answer: False

Thermobarometry and thermochronology answer different questions. Thermobarometry uses the chemical compositions of coexisting mineral pairs to determine the P-T conditions at which those minerals last equilibrated (crystallization or peak metamorphism conditions). Thermochronology uses the accumulation of radiogenic daughter isotopes to determine when a mineral cooled through its closure temperature. They are complementary techniques: thermobarometry constrains the P-T history, thermochronology constrains the timing and rate of that history. Combining them yields the full P-T-t path.

Question 5

How does the chemical composition of a zoned garnet crystal record a P-T path, and why do the core and rim record different conditions?

Model answer: Garnet grows incrementally as a rock is buried and metamorphosed. The core crystallizes at the earliest (lowest P-T) conditions and its Fe/Mg and Ca/Mn ratios are set by equilibrium with the surrounding mineral assemblage at that point. As burial continues and P and T rise, new garnet overgrows the existing crystal as a rim, equilibrating with the new conditions. Diffusion within the garnet interior is slow enough that the core composition is largely preserved rather than re-equilibrating. The result is a chemical profile across the crystal — core to rim — that archives the changing P-T conditions during the rock's burial and heating history.

Zoned garnets are natural recorders of metamorphic history because garnet is a slow diffuser at moderate temperatures — it preserves compositional gradients that would be erased in faster-diffusing minerals. A geologist can measure a microprobe transect across a garnet (measuring Fe, Mg, Ca, Mn at intervals from core to rim) and decode the P-T conditions at each growth stage. Combining multiple garnet zones with independent barometers (e.g., Al-in-hornblende) and geochronology can reveal a detailed history of burial, peak metamorphism, and exhumation across tens of millions of years of tectonic activity.